[LLVM] Remove the requirement for named pipe in jobserver (#169154)

Remove the requirement that the jobserver "fifo" is actually a named
pipe. Named pipes are essentially stateless, and therefore carry a high
risk of a killed process leaving the server with no tokens left, and no
clear way to reclaim them. Therefore, multiple jobserver implementations
use FUSE instead:

- [nixos-jobserver](NixOS/nixpkgs#314888) (WIP)
uses simple file on FUSE

- [steve](https://gitweb.gentoo.org/proj/steve.git) uses a character
device via CUSE

- [guildmaster](https://codeberg.org/amonakov/guildmaster) uses a
character device via CUSE

This is compatible with GNU make and Ninja, since they do not check the
file type, and seems to be the only solution that can achieve state
tracking while preserving compatibility.

CC @amonakov

---------

Signed-off-by: Michał Górny <mgorny@gentoo.org>

Author: mgorny

llvm/lib/Support/Unix/Jobserver.inc

@@ -19,14 +19,6 @@
 #include <unistd.h>
 
 namespace {
-/// Returns true if the given file descriptor is a FIFO (named pipe).
-bool isFifo(int FD) {
-  struct stat StatBuf;
-  if (::fstat(FD, &StatBuf) != 0)
-    return false;
-  return S_ISFIFO(StatBuf.st_mode);
-}
-
 /// Returns true if the given file descriptors are valid.
 bool areFdsValid(int ReadFD, int WriteFD) {
   if (ReadFD == -1 || WriteFD == -1)
@@ -75,7 +67,7 @@ JobserverClientImpl::JobserverClientImpl(const JobserverConfig &Config) {
   case JobserverConfig::PosixFifo:
     // Open the FIFO for reading. It must be non-blocking and close-on-exec.
     ReadFD = ::open(Config.Path.c_str(), O_RDONLY | O_NONBLOCK | O_CLOEXEC);
-    if (ReadFD < 0 || !isFifo(ReadFD)) {
+    if (ReadFD < 0) {
       if (ReadFD >= 0)
         ::close(ReadFD);
       ReadFD = -1;

llvm/unittests/Support/JobserverTest.cpp

@@ -14,6 +14,7 @@
 #include "llvm/Support/Jobserver.h"
 #include "llvm/Config/llvm-config.h"
 #include "llvm/Support/Debug.h"
+#include "llvm/Support/FileUtilities.h"
 #include "llvm/Support/Parallel.h"
 #include "llvm/Support/Program.h"
 #include "llvm/Support/ThreadPool.h"
@@ -221,6 +222,40 @@ TEST_F(JobserverClientTest, UnixClientFifo) {
   EXPECT_TRUE(S1.isValid());
 }
 
+TEST_F(JobserverClientTest, UnixClientNonFifo) {
+  // This test verifies that non-FIFO jobservers can be used, such as steve
+  // or guildmaster.
+  SmallString<64> F;
+  std::error_code EC =
+      sys::fs::createTemporaryFile("jobserver-test", "nonfifo", F);
+  ASSERT_FALSE(EC);
+  FileRemover Cleanup(F);
+
+  // Intentionally inserted \t in environment string.
+  std::string Makeflags = " \t -j4\t \t--jobserver-auth=fifo:";
+  Makeflags += F.c_str();
+  ScopedEnvironment Env("MAKEFLAGS", Makeflags.c_str());
+
+  JobserverClient *Client = JobserverClient::getInstance();
+  ASSERT_NE(Client, nullptr);
+
+  // Get the implicit token.
+  JobSlot S1 = Client->tryAcquire();
+  EXPECT_TRUE(S1.isValid());
+  EXPECT_TRUE(S1.isImplicit());
+
+  // File is empty, next acquire fails.
+  JobSlot S2 = Client->tryAcquire();
+  EXPECT_FALSE(S2.isValid());
+
+  // Release does not write to the file for the implicit token.
+  Client->release(std::move(S1));
+
+  // Re-acquire the implicit token.
+  S1 = Client->tryAcquire();
+  EXPECT_TRUE(S1.isValid());
+}
+
 #if LLVM_ENABLE_THREADS
 // Unique anchor whose address helps locate the current test binary.
 static int JobserverTestAnchor = 0;